1. Field of the Invention
The present invention relates to a cooling structure in a parallel multiple-cylinder engine especially having a cylinder-integrated crankcase of aluminum die-cast.
2. Description of the Related Art
Conventionally, in an engine having a water-cooling type cooling device as a cooling device of a four-cycle or two-cycle engine, cooling water is supplied to a water jacket formed to surround a cylinder by a water pump. On this occasion, various kinds of contrivance are made to enhance cooling performance, and decrease the size.
For example, in a V-type engine described in Japanese Patent Application Laid-open No. 2-245423, an extended part extending in the direction of a crankshaft is formed at an end surface of a cylinder body which is offset to the other end side in an axial direction of the crankshaft, a step part is formed between this extended part and the cylinder body, and a communication water passage which guides cooling water to a water pump is disposed at this step part.
In a cooling device described in Japanese Patent Application Laid-open No. 61-265343, a partition wall is provided in a water jacket, and a flow passage in a proper shape is formed in this partition wall.
Further, in a water-cooling type multi cylinder engine described in Japanese Utility Model Application Laid-open No. 4-27139, a main water passage passing in a cylinder arrangement direction below each water chamber is provided in a wall body of a cylinder block, and the main water passage and each of the water passages are made to communicate each other separately for each cylinder.
Incidentally, in a parallel multi-cylinder engine of this kind, there is an engine in which a crankcase is cast to be integral with a cylinder of aluminum die cast, with the cylinder made a plated cylinder, the cylinder has an angle from verticality with respect to the mating surface of the crankcase, and a water passage is included on a mating surface with the cylinder head.
In an engine of this type, the lower side of the water jacket at an intake side is undercut due to casting, and considerable useless thickness is included as it is. Since the cylinder surface slides with a piston relatively at a high speed, a casting cavity, a casting sink, a crack or the like which is the drawback of the die cast, becomes a fatal problem for an engine as it is. Useless thickness not only becomes the largest cause of such problems, but also becomes the cause of an increase in unnecessary weight as a matter of course.
Meanwhile, the water pump is usually driven by a countershaft or a crankshaft, and is located at a rear side from the cylinder axis. In this case, piping of a water passage to the cylinder has the shortest distance when connected to an intake side of the cylinder, and thereby, favorable pump efficiency can be obtained.
However, the lower side of the mounting surface of the water inlet also becomes undercut and has useless thickness due to casting, and therefore, the cylinder with the aforementioned structure is inconvenient.
As the conventional countermeasure, piping is arranged around the cylinder exhaust side, for example. In this case, however, the length of the passage becomes long, and not only the pump loss becomes large, but also a space for leading the piping to an outside of the cylinder or a lower part of the case becomes necessary. In the case with a faring, the width becomes wider correspondingly, which causes increase in air resistance, reduction in performance and fuel efficiency. In the case without a fairing, there is the possibility of breakage when falling down as it is.
In the case of an ordinary cylinder, with an even number of cylinders, it is not efficient in securing a passage to connect the passage by striding a boss for a cylinder head fastening screw between bores, and hence, the passage is actually connected at the side of either one of them. Therefore, cooling of each cylinder cannot be performed uniformly, thus causing a variation of the combustion condition, and hence, reduction in engine performance is caused.